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Optimizing the Polyethylene Oxide and Polypropylene Oxide Contents in Diethylenetriamine-Based Surfactants for Destabilization of a Water-in-Oil Emulsion Yuming Xu,* Jiangying Wu, Tadeusz Dabros, and Hassan Hamza CANMET Energy Technology CentresDevon, 1 Oil Patch Drive, Devon, Alberta T9G 1A8, Canada
Johann Venter Champion Technology Inc., Fresno, Texas 77245 Received September 10, 2004. Revised Manuscript Received February 7, 2005
In this work, we used polyoxyalkylenated diethylenetriamine (DETA) demulsifier with various polyethylene oxide (PEO) and polypropylene oxide (PPO) contents to destabilize a stable waterin-oil emulsion. The demulsifiers were characterized by the relative solubility number (RSN). The efficiency of emulsion destabilization was measured by the degree of separation of the oil and water phases. It was found that the destabilization of an emulsion is closely correlated with the PO and EO numbers. When the PO number in a molecule is much greater than the EO number, the surfactant gives a very low oil resolution rate, requires high dosages, and produces a stable middle phase. When a surfactant contains more EO than PO, it gives a high oil resolution rate at a low dosage, but it is easily overdosed, and some surfactants of this type (high molecular weight) also produce a stable middle emulsion phase. When the PO and EO numbers in the surfactant are close to equal, the surfactant breaks the emulsion rapidly at a very low dosage, does not show overdosing at very high dosage, and does not produce a stable middle phase. Therefore, surfactants with balanced PO and EO numbers give optimal performance.
Introduction Stable water-in-oil emulsions are frequently encountered in the oil industry. Destabilization of such emulsions is an important step in oil production. Although there are many different methods for destabilizing water-in-oil emulsions, adding surfactant demulsifier is still one of the most common methods. Of the different types of surfactants, nonionic surfactants are currently most widely used in the oil industry. The main components in nonionic surfactants are polyethylene oxide (PEO) groups and polypropylene oxide (PPO) groups. Generally, the PO blocks provide the hydrophobic group of the surfactant, and the EO blocks function as the hydrophilic group. By varying the number and array of the PO and EO groups, surfactants with different characteristics can be synthesized. The hydrophile-lipophile balance (HLB) is one of the most important characteristics of a surfactant. Due to the way the HLB concept was developed, it is difficult to measure the HLB value of a surfactant. Alternatively, relative solubility numbers (RSNs) can be used to represent the hydrophobicity of the surfactant. Our previous research has shown a linear correlation between HLB and RSN for surfactants in the same family.1 The RSN value of a surfactant is directly related to its PO and EO composition. Linear correlations * To whom correspondence should be addressed. Phone: (780) 9878637. Fax: (780) 987-8676. E-mail:
[email protected].
between the RSN value and EO number have been reported for surfactant families containing the same number of POs.2 When a surfactant is used to destabilize a water-inoil emulsion, its performance is directly affected by characteristics that include surface, interfacial, and chemical properties. In addition to investigations on the effects of surfactant interfacial properties, rheological behavior, and partitioning behavior on demulsification,3-8 previous researchers have also tried to correlate demulsifier efficiency with surfactant chemical properties such as HLB or composition.9-14 From their studies on the (1) Wu, J.; Xu, Y.; Dabros, T.; Hamza, H. Colloids Surf., A 2004, 232, 229-237. (2) Xu, Y.; Wu, J.; Dabros, D.; Hamza, H.; Wang, S.; Bidal, M.; Venter. J.; Tran. T. Can. J. Chem. Eng. 2004, 82, 829-835. (3) Mohammed, R. A.; Baily, A. I.; Lickham, P. F.; Taylor, S. E. Colloids Surf., A 1993, 80, 237-242. (4) Mohammed, R. A.; A. I. Baily, Lickham, P. F.; Taylor, S. E. Colloid Surf., A 1994, 83, 261-271. (5) Kim, Y. H.; Wasan, D. T. Ind. Eng. Chem. Res. 1996, 35, 11411149. (6) Krawczyk, M. A.; Wasan, D. T.; Shetty, C. S. Ind. Eng. Chem. Res. 1991 30, 367-375. (7) Djuve, J.; Yang, X.; Fjellanger, I. J.; Sjoblom, J.; Pelizzetti, E. Colloid Polym Sci. 2001, 279, 232. (8) Zapryanov, Z.; Malhotra, A. K.; Aderangi, N.; Wasan, D. T. Int. J. Multiphase Flow 1983, 9, 105. (9) Cooper, D. G.; Zajic, J. E.; Cannel, E. J.; Wood. J. W. Can. J. Chem. Eng. 1980, 58, 576-579. (10) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Lu, J. R. J. Colloid Interface Sci. 1990, 139, 128-138. (11) Marquez-Silva, R. L.; Key, S.; Marini, J.; Guzman, C.; Buitriago, S. SPE 37271 1997, 601-607.
10.1021/ef0497661 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/12/2005
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demulsification of a water-in-crude oil emulsion using surfactants, Cooper et al. and Averyard et al. claimed that the demulsification efficiency of the surfactant was correlated with its HLB value.9,10 Marquez-Silva et al. proposed an empirical model to correlate the nature of the crude oil, the associated water salinity, and the demulsifier hydrophilicity.11 Shetty et al. found that a surfactant containing a high percentage of hydrophilic groups and having a low molecular weight can function as a good demulsifier.12 Zaki et al. studied the demulsification efficiency of amine-based surfactants with different amine groups and EO numbers.13,14 They used only three surfactants in each group. On the basis of the limited number of samples they found that a lower HLB surfactant gave a better demulsification efficiency. However, their surfactant samples contained only very short PO and EO chains and are not representative of the industrial demulsifiers. Recently, Sun et al. investigated the influence of demulsifier structure on the interfacial dilational properties of an oil-water interface using one pair of straight and branched demulsifiers and some surface-active fractions from petroleum.15 On the basis of the dilational property measurements for the system with demulsifier, the surface-active fraction, and the mixture of both, they claimed that it is the molecular size of the surface-active fraction that influences the adsorption to the interface. However, this work did not explain how the demulsifier can destabilize the emulsion since the surface-active fractions from oil are already at the interface to stabilize the emulsion, and the influence of the demulsifier structure was not addressed. Most recently, Wu et al. reported that the performance of commercial demulsifiers is correlated with both their RSN and molecular weights and an optimal RSN value in terms of demulsification was observed for surfactants in the same family.16 However, the detailed chemical structures in the commercial demulsifiers used in this work were not available, and only the RSN and molecular weight were reported. It is, therefore, necessary to gain knowledge about the influence of the chemical structure of a commercial-type demulsifier on the demulsification efficiency. In this work, we studied the demulsification of a water-in-oil emulsion using diethylenetriamine (DETA)based surfactants with systematic variations in their PO and EO compositions. The objective of this study was to investigate the effect of PO and EO composition on demulsification performance. Experimental Section Materials. All DETA-based surfactant samples were supplied by Champion Technology Inc., Fresno, TX. The surfactants used in this work are all block copolymers which were synthesized by reacting the acceptor diethylenetriamine with propylene oxide first, followed with ethylene oxide. There are five arms in each molecule, and each arm has a block PO and (12) Shetty, C. S.; Nikolov, A. D.; Wasan, D. T. J. Dispersion Sci. Technol. 1992, 13, 121-129. (13) Zaki, N. N.; Abdel-Raouf, M. E.; Abdel-Azim, A. A. Chem. Mon. Austria 1996, 127, 621-629. (14) Zaki, N. N.; Maysour, N. E-S.; Abdel-Azim, A. A. Pet. Sci. Technol. 2000, 18, 1009-1025. (15) Sun, T.; Zhang. L.; Wang, Y.; Zhao, S.; Peng, B.; Li, M.; Yu, J. J. Colloid Interface Sci. 2002, 255, 241-24. (16) Wu, J.; Xu, Y.; Dabros, T.; Hamza, H. Energy Fuels 2003, 17, 1554-1559.
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Figure 1. Chemical structure of a DETA-based block copolymer. Table 1. Demulsifiers Used in This Work with Selected Chemical Properties PO number
EO number
molecular weight
RSN
155
10.8 22.7 36.1 51.1 68.2 87.7 110.1 136.4 170.0 12.6 20.1 28.4 37.9 48.7 61.2 75.8 93.0 113.6 9.4 15.0 21.3 28.3 36.4 45.8 56.7 69.5
9722 10248 10836 11498 12248 13105 14094 15248 15017 5733 6059 6427 6844 7320 7869 8510 9268 10177 4295 4539 4814 5126 5482 5893 6372 6939
7.6 8.5 9.8 11.5 12.2 14.9 18.0 22.5 30.4 9.8 11.0 12.3 14.3 16.4 19.0 21.8 27.1 29.2 11.1 12.2 13.0 14.9 15.7 18.6 21.4 24.7
126 86
64
IFTa (mN/m) 16.2
9.2 5.5 13.1 11.3 9.5 7.7 5.7 4.5 3.0 2.3 4.3 12.8
5.2 4.3
a
IFT values are for the interfacial tension between 5% bitumen in toluene and DI water, with 200 ppm demulsifier in the oil phase.
a block EO [DETA-(PO)x-(EO)y]. The molecular structure of the block copolymer is shown in Figure 1. With variation of PO and EO numbers, surfactants with different hydrophobicities were obtained. In this work, four series of block surfactants were used. Each series contains the same number of POs in the PPO block but different numbers of EOs in the surfactant molecules. The PO numbers in the copolymer molecules were determined by reacting the hydroxyls in the sample with excess acetic anhydride followed by back-titration with KOH to determine the hydroxyl concentration. The average EO number in a polymer molecule was determined from the ethylene oxide mass composition. Table 1 lists all demulsifier samples used in this work along with some selected chemical properties. The bitumen sample used in this work was Syncrude coker feed bitumen. The toluene used for making bitumen solution was industry grade, supplied by Fisher Scientific. The toluene and ethylene glycol dimethyl ether used for RSN titration were HPLC grade, purchased from Aldrich Chemicals.
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Preparation of a Stable Emulsion. The oil phase used for preparation of the emulsion was a 5 wt % bitumen in toluene solution, which was prepared by dissolving a preselected amount of bitumen in toluene. The bitumen solution was centrifuged under 4000g force for 30 min to remove fine solid particles before it was used to make the emulsion. Equal masses of the bitumen solution and a 0.01 M NaCl aqueous solution were placed in a glass jar and mixed by a polytron homogenizer (model PT 1300D) at 20000 rpm for 3 min. A stable water-in-oil emulsion was thus obtained. The emulsion prepared in this way was stable for weeks without the appearance of phase separation. Demulsification Tests. The emulsion described above was transferred to a graduated test tube (about 14 mL total volume) and allowed to age for 10 min by placing the test tube in a water bath set at 60 °C. A predetermined amount of demulsifier was then added to the emulsion using a micropipet, and the test tubes were stopped and shaken for 1 min to mix the demulsifier thoroughly with the emulsion. The tubes were then returned to the water bath, and phase separation was monitored. During the settling, the interface between the emulsion and water phase was difficult to observe due to the presence of the middle phase (concentrated emulsion). However, the interface between the top oil phase and emulsion was sharp. The oil phase separated at the top of the tube appeared black and contained less than 0.2 wt % water as measured by Karl Fischer titration. Therefore, the position of the oil/emulsion interface was recorded as a function of time to obtain the volume of oil resolved. The volume of separated oil phase can be read accurately to (0.05 mL since the test tube is graduated to 0.1 mL. Therefore, the relative errors in the estimation of the volume of the resolved oil phase are from 5% for low-resolution tests (1 mL of oil, or 15% separation) to less than 1% for highresolution tests (>5 mL of oil, or >70% separation). The performance of a surfactant was measured by the parameter oil resolution (Roil), which is defined as
Roil (vol %) ) Voil/V°oil × 100
(1)
where Voil is the volume of the separated oil phase and V°oil is the known volume of oil in the initial emulsion. After the contents settled at 60 °C for 1 h the test tube was centrifuged at about 400g for 5 min. After centrifugation the volume of the middle phase was measured. The reason for centrifugation is that, sometimes, large water droplets were seen within the middle phase, which made it difficult to estimate the volume of the middle-phase emulsion. The large water droplets (in millimeter size) do not represent a stable emulsion and are very easy to separate. With a slight external force such as stirring or gentle centrifugation they settle to the water phase. Since the final interface between the middlephase emulsion and water is sometimes curved, the estimation of the volume of the middle phase is less accurate than that for the oil phase. We estimate that the errors associated with the measurement of the volume of the middle emulsion phase are about 0.1-0.2 mL. RSN Measurement. The RSN values of the surfactants were measured by the newly developed method using ethylene glycol dimethyl ether and toluene as titration solvents. In this method, 1 g of surfactant was dissolved in 30 mL of solvent consisting of toluene and ethylene glycol dimethyl ether and the resultant solution was titrated with DI water till the solution became persistently turbid. The volume of water (in mL) titrated in was recorded as the RSN number. The details of the method can be found in ref 1. The reproducibility of the RSN measurement was found to be excellent. As reported in ref 1 the standard deviation from five separated measurements for a surfactant with RSN ) 13.7 was less than 0.06.
Figure 2. RSN value as a function of EO number for DETAbased block copolymers.
Figure 3. Oil resolution as a function of settling time for block copolymers containing 64 POs and different numbers of EOs.
Results and Discussion RSN of the Surfactants. The four series of DETAbased nonionic surfactants used in this work have approximate PO numbers of 64, 86, 126, and 155, respectively. Since each molecule has five arms, each arm in these four series of molecules has a corresponding PO number of 13, 17, 25, and 31, respectively. The EO number in the molecule varies significantly for each fixed-PO-number molecule. RSN values for all of the surfactants were measured, and the results are linearly correlated with the EO number at each fixed PO number, as shown in Figure 2. The graph indicates that, at the fixed PO number, the RSN value of the surfactant increases with the EO number. On the other hand, when the EO number is fixed, the RSN value decreases with increasing PO number. This clearly indicates the effect of EO and PO numbers on the hydrophobicity of the surfactant: an increasing EO number enhances the hydrophilicity of the surfactant, while an increasing PO number enhances its hydrophobicity. Effect of EO Block Length on Demulsification Efficiency. On the basis of the results of pretests at various dosages, a dosage of 200 ppm on the basis of the oil phase was selected for the tests. At this dosage the demulsification performance of different surfactants can be distinguished easily. Figure 3 shows the oil resolution for the series of surfactants containing 64 POs. The EO number varies from 10 to 71 in the surfactant molecules. As the figure shows, when the molecule contains only 10 EOs, the oil resolution is very slow. After 10 min, only about 10% of the oil was separated. After 60 min, the oil resolution leveled off at about 40%. As the EO number increases, the oil resolution speeds up and emulsion breakage appears to become more complete. When the surfactant molecule contains more than 58 EOs, the emulsion is broken in a few minutes and 100% oil resolution is obtained.
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Figure 4. Oil resolution as a function of settling time for block copolymers containing 86 POs and different numbers of EOs.
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Figure 6. Oil resolution as a function of settling time at different dosages for a copolymer containing 64 POs and 15 EOs.
Figure 5. Oil resolution as a function of settling time for block copolymers containing 155 POs and different numbers of EOs.
Figure 4 shows the oil resolution results for a surfactant series containing 86 POs. The EO number in the molecules varies from 13 to 115. From Figure 4 it can be seen that the oil resolution also increases with increasing the EO number. When the EO number approaches about 80, the oil resolution reaches almost 100%. A slight decrease in oil resolution is observed when the EO number exceeds 90. Figure 5 shows a similar oil resolution graph for a surfactant series containing 155 POs and 11-138 EOs. Again, at low EO content, the oil separates more slowly, and the oil resolution rate increases with increasing EO. The oil resolution reaches 100% when the molecules contain about 100 EOs. Effect of PO and EO Blocks on Demulsification Efficiency. The results reported in Figures 3-5 indicate a general trend that demulsification efficiency increases with increasing EO block length until the EO block reaches a certain length. The optimal EO number to reach maximum emulsion breakage was also dependent on the PO number. To further investigate the effect of both EO and PO, we carried out other series of demulsification tests. The tests were done for different dosages with three scenarios in terms of relative PO and EO numbers: PO . EO, PO ≈ EO, and PO < EO, as discussed below. (1) PO . EO. When the PO number is much greater than the EO number, the RSN values are small and the surfactant molecules are hydrophobic. These surfactants did not break the emulsion efficiently at the dosage of 200 ppm as can be seen from Figures 3-5. Figure 6 shows the oil resolution as a function of time at different dosages for a selected surfactant containing 64 POs and 15 EOs. The dosages range from 100 to 2000 ppm. As the graph shows, in the dosage range of 100-300 ppm the oil resolution is very slow. However, as the dosage
Figure 7. Oil resolution as a function of settling time at different dosages for copolymers containing (a) 64 POs and 58 EOs and (b) 155 POs and 138 EOs.
increases to above 800 ppm, the oil resolution becomes very rapid and the emulsion is completely broken in less than 5 min. When the dosage reaches 2000 ppm, the oil resolution is still rapid and no overdosing is observed. Therefore, a high dosage of this type of surfactant is required. (2) PO ≈ EO. When the PO number and EO number in the surfactant molecules are almost the same, the hydrophilic and hydrophobic properties are well balanced. The RSN numbers are in the range of 20-25. Figure 7 shows the oil resolution curves at different dosages for two surfactants that contain similar numbers of POs and EOs. Surfactant 1 contains 64 POs and 58 EOs, and surfactant 2 has 155 POs and 138 EOs. As the graphs show, both surfactants can break the emulsion even at very low dosages. At a dosage of 50 ppm, the oil resolution is almost 100% in less than 5 min. Furthermore, the surfactants function over a very wide range of dosages. In the dosage range between 50 and 2000 ppm, the surfactants function almost identically and no overdosing is observed. These results indicate that this type of surfactant breaks the emulsion very
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Figure 9. Oil resolution rate as a function of PO/EO ratio.
Figure 10. Oil resolution at 3 min as a function of RSN for three series of copolymers. Figure 8. Oil resolution as a function of settling time at different dosages for copolymers containing (a) 126 POs and 170 EOs and (b) 86 POs and 115 EOs on each arm.
efficiently and can serve as a useful guide in selecting optimal emulsion breakers. (3) PO < EO. In the third case, the surfactants containing more EOs than POs are hydrophilic, with high RSN values. Figure 8 shows the oil resolution behavior for two selected surfactants, one containing 126 POs and 170 EOs, and the other containing 86 POs and 115 EOs. The graphs indicate that the two surfactants can break the emulsion at a low dosage, 50 ppm. However, as the dosage increases the demulsification performance weakens, indicating overdosing. Maximum performance is observed in a narrow range of dosages. Therefore, this type of surfactant is difficult to use as an emulsion breaker. Effect of the RSN. The discussion above clearly indicates that the oil resolution or demulsification performance of the surfactants is dependent on the PO and EO balance, which determines the RSN of the surfactants. To correlate the demulsification performance with the RSN or the PO-to-EO ratio (PO/EO), we chose the oil resolution percentage at 3 min as a measure of the oil resolution rate or demulsification efficiency. Figure 9 shows the oil resolution at 3 min (dosage 200 ppm) as a function of PO/EO ratio. The graph clearly demonstrates that at a PO/EO ratio of about 1, the oil resolution is highest, almost 100%. As the ratio increases the demulsification performance decreases. Figure 10 shows the oil resolution at 3 min as a function of the RSN for three series of surfactants. In the RSN range below 18 the oil resolution for all surfactants is lower than 80% and increases with the RSN. When the RSN values are 20 and above, the oil resolution reaches a maximum and levels off. Oil resolution is used as our primary criterion for evaluating demulsifier performance in this study; how-
ever, other factors have to be considered as well. It was observed that, for some surfactants, after settling, the emulsion had broken and separated into oil and water phases, but for some other surfactants, a creamy middle phase had formed between the water and oil. This creamy middle phase was a concentrated emulsion and was very difficult to break further. The photographs in Figure 11 show (a) the complete separation of water and oil when the surfactant with 64 POs and 58 EOs was applied, (b) the middle creamy phase after 1 h of settling when the surfactant with 86 POs and 115 EOs was applied, and (c) the middle phase after centrifugation at 400g for 5 min for the same sample as in (b). From Figure 11 it can be seen that, in case a, when the surfactant with balanced POs and EOs was used, the emulsion was broken completely without formation of the middle phase while, in the second case, when using the surfactant with higher EO contents, the middle phase formed. Therefore, in addition to the oil resolution rate, we have also recorded the volume of the middle phase after centrifugation at about 400g for 5 min. The volume percentage of the middle phase after centrifugation is used as a second parameter to evaluate the performance of a surfactant. Figure 12 shows the volume percentage of the middle phase for the DETA surfactants as a function of the RSN. As the graph indicates, for each series of surfactants, the middle phase forms at low RSN. The lower the RSN value, the greater the volume of the middle phase. When the RSN increases to a specific value, the middle phase disappears. This specific RSN value is different for each surfactant series, and is dependent on the PO number. For the surfactant series containing 64 POs, no middle phase forms in the entire RSN range above 18. For the surfactant series containing 86 POs, the middle phase forms at low and high RSN values, while no middle phase is observed in the RSN range of 15-25. However, for the surfactant series containing 155 POs, only a very narrow RSN range exists, 12-15, in which no middle
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Figure 13. Oil resolution at 3 min as a function of interfacial tension for DETA surfactants.
Figure 11. Photographs showing the middle creamy phase after settling and centrifugation: (a) PO13, EO12, after 1 h of settling; (b) PO17, EO23, after 1 h of settling; (c) the same sample in (b) after 5 min of centrifugation.
change the interfacial properties, such as interfacial tension, film pressure, or interfacial viscosity, so that the film thinning (drainage) rate is enhanced and the stability of the film is reduced.3-5,19-22 We measured the interfacial tension between a bitumen-in-toluene solution and water with 200 ppm ((3-5) × 10-8 mol/L of oil phase) demulsifier in the oil phase for selected demulsifiers. The interfacial tension values are reported in Table 1 and were also correlated with the oil resolution at 3 min as shown in Figure 13. The graph shows a fairly good correlation between oil resolution and interfacial tension; the oil resolution increases with a decrease in the interfacial tension. This indicates that the demulsifier reduced the interfacial tension and enhanced the emulsion breaking. However, it should be pointed out that lowering the interfacial tension is not the only factor for destabilization of the emulsion. Other parameters such as interfacial elasticity, partitioning behavior, and film pressure are also very important in destabilization of the emulsion. Conclusions
Figure 12. Volume percentage of the middle phase emulsion as a function of RSN for three series of copolymers.
phase is formed. These results indicate that as the PO number increases the RSN range for complete emulsion breaking becomes narrower. Effect of Interfacial Tension. The mechanism of destabilization of a water-in-crude oil emulsion by adding surfactants is of great theoretical and practical interest.3-7,17-20 It is generally believed that the demulsifier molecules adsorb onto the oil/water interface and (17) Wasan, D. T.; Nikolov, A. Proceedings of the First World Congress on Emulsions, Paris, France, Oct. 19-22, 1993. (18) Kilpatrick, P. K.; McLean, J. D. J. Colloid Interface Sci. 1997, 189, 242-251. (19) Young, H. K.; Wasan, D. T. Ind. Eng. Chem. Res. 1996, 35, 1141-1149. (20) Tambe, D.; Paulis, J. A.; Sharma, M. M. J. Colloid Interface Sci. 1995, 171, 463-469.
(1) The demulsification performance of DETA-based copolymers is correlated with their PO and EO compositions or RSN numbers. For a surfactant series containing a fixed number of POs the oil resolution rate increases with the EO number in the molecules until leveling off at a high RSN. (2) PO and EO numbers are interrelated in affecting the demulsification performance. When a surfactant contains many more POs than EOs, it gives a low oil resolution rate at lower dosages, requires high dosages to break the emulsion, and produces a stable middle phase. A surfactant containing more EOs than POs results in a high oil resolution rate and requires only a low dosage. However, this type of surfactant is easily overdosed, and some of them produce a stable middlephase emulsion, especially those with high PO numbers (large molecular weight). When the PO and EO numbers in a surfactant are close to equal, the surfactant breaks the emulsion very rapidly and requires only a very low dosage, but does not show overdosing at very high dosage and does not produce a stable middle phase. Therefore, the POs and EOs should be well balanced to give optimal performance. EF0497661 (21) Eley, D. D.; Hey, M. J.; Lee, M. A. J. Colloid Interface Sci. 1987, 25, 173-182. (22) Singh, B. P. Energy Sources 1994, 16, 377-385.
